Semiconductor device using a nitride semiconductor

ABSTRACT

A semiconductor device includes: a first semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1); a first conductivity type or non-doped second semiconductor layer represented by a composition formula Al y Ga 1-y N (0≦y≦1, x&lt;y) and formed on the first semiconductor layer; a second conductivity type third semiconductor layer represented by a composition formula Al x Ga 1-x N (0≦x≦1) and selectively formed on the second semiconductor layer; a gate electrode formed on the third semiconductor layer; a source electrode electrically connected to the second semiconductor layer; and a drain electrode electrically connected to the second semiconductor layer. The distance between the drain electrode and the third semiconductor layer is longer than the distance between the source electrode and the third semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of Ser. No. 10/831,130,filed Apr. 26, 2004 and is based upon and claims the benefit of priorityunder 35 U.S.C. §119 to Japanese Patent Application No. 2004-54330,filed on Feb. 27, 2004; the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, for example, toan insulating gate type field effect transistor using a nitridesemiconductor.

2. Related Background Art

Since a nitride semiconductor device using gallium nitride (hereinafterreferred to simply as GaN) has a large band gap as compared with asemiconductor device using silicon (Si), the semiconductor device has ahigh critical electric field, and from this characteristic, asmall-sized device having a high breakdown voltage is easily realized.Accordingly, in a semiconductor for electric power control, a lowon-resistance is achieved, and a device having a low loss can berealized. Above all, in a field effect transistor using an AlGaN/GaNheterostructure (hereinafter referred to simply as an HFET (aheterostructure field effect transistor), satisfactory characteristicscan be expected with a simple device structure. A gate electrode in ANHFET has a Schottky gate structure forming a Schottky junction with anAlGaN layer. Moreover, a conventional GaN-based HFET is a normallyon-type device in which a current flows between a source and a drain,when a drain voltage is applied at a gate voltage of zero.

However, in general, the Schottky gate structure has a problem that aleak current increases, when a gate leak current is large and devicetemperature rises. A normally-on type device has a problem that a largecurrent flows at the moment at which a power to a circuit is turned on,and this sometimes results in destruction of the device.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda semiconductor device comprising:

a first semiconductor layer represented by a composition formulaAl_(x)Ga_(1-x)N (0≦x≦1);

a first conductivity type or non-doped second semiconductor layerrepresented by a composition formula Al_(y)Ga_(1-y)N (0≦y≦1, x<y) and isformed on the first semiconductor layer;

a second conductivity type third semiconductor layer represented by acomposition formula Al_(x)Ga_(1-x)N (0≦x≦1) and is selectively formed onthe second semiconductor layer;

a gate electrode formed on the third semiconductor layer;

a source electrode electrically connected to the second semiconductorlayer; and

a drain electrode electrically connected to the second semiconductorlayer;

wherein the distance between the drain electrode and the thirdsemiconductor layer is longer than the distance between the sourceelectrode and the third semiconductor layer.

According to a second aspect of the present invention, there is provideda semiconductor device comprising:

a first semiconductor layer represented by a composition formulaAl_(x)Ga_(1-x)N (0≦x≦1);

a first conductivity type or non-doped second semiconductor layerrepresented by a composition formula Al_(y)Ga_(1-y)N (0≦y≦1, x<y) and isformed on the first semiconductor layer;

a second conductivity type third semiconductor layer represented by acomposition formula Al_(x)Ga_(1-x)N (0≦x≦1) and is selectively formedabove the second semiconductor layer;

a gate insulator formed on the third semiconductor layer;

a gate electrode formed on the gate insulator;

a source electrode electrically connected to the second semiconductorlayer; and

a drain electrode electrically connected to the second semiconductorlayer.

According to a third aspect of the present invention, there is provideda semiconductor device comprising:

a first semiconductor layer represented by a composition formulaAl_(x)Ga_(1-x)N (0≦x≦1);

a first conductivity type or non-doped second semiconductor layerrepresented by a composition formula Al_(y)Ga_(1-y)N (0≦y≦1, x<y),formed on the first semiconductor layer and having a concave portion;

a second conductivity type third semiconductor layer represented by acomposition formula Al_(x)Ga_(1-x)N (0≦x≦1) and is selectively formedabove a bottom surface of the concave portion of the secondsemiconductor layer;

a gate electrode formed on the third semiconductor layer;

a source electrode electrically connected to the second semiconductorlayer; and

a drain electrode electrically connected to the second semiconductorlayer.

According to a fourth aspect of the present invention, there is provideda semiconductor device comprising:

a first conductivity type or non-doped first semiconductor layerrepresented by a composition formula Al_(y)Ga_(1-y)N (0≦y≦1, x<y), thefirst semiconductor layer having a first surface and a second surfaceopposite to the first surface, and further having a concave portion onthe side of the first surface;

a second conductivity type second semiconductor layer represented by acomposition formula Al_(x)Ga_(1-x)N (0≦x≦1) and is formed on the secondsurface of the first semiconductor layer;

a gate electrode formed above a bottom surface of the concave portion ofthe first semiconductor layer;

a source electrode electrically connected to the first semiconductorlayer; and

a drain electrode electrically connected to the first semiconductorlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing a first embodiment of asemiconductor device according to the present invention;

FIG. 2 is a sectional view schematically showing a first modification ofthe semiconductor device shown in FIG. 1;

FIG. 3 is a sectional view schematically showing a second modificationof the semiconductor device shown in FIG. 1;

FIG. 4 is a sectional view schematically showing a second embodiment ofa semiconductor device according to the present invention;

FIG. 5 is a sectional view schematically showing a modification of thesemiconductor device shown in FIG. 4;

FIG. 6 is a sectional view schematically showing a third embodiment of asemiconductor device according to the present invention;

FIG. 7 is a sectional view schematically showing a first modification ofthe semiconductor device shown in FIG. 6;

FIG. 8 is a sectional view schematically showing a second modificationof the semiconductor device shown in FIG. 6;

FIG. 9 is a sectional view schematically showing a fourth embodiment ofa semiconductor device according to the present invention;

FIG. 10 is a sectional view schematically showing a modification of thesemiconductor device shown in FIG. 9;

FIG. 11 is a perspective view schematically showing a fifth embodimentof a semiconductor device according to the present invention;

FIG. 12 is a perspective view schematically showing a modification ofthe semiconductor device shown in FIG. 10;

FIG. 13 is a sectional view schematically showing a sixth embodiment ofa semiconductor device according to the present invention;

FIG. 14 is a sectional view schematically showing a first modificationof the semiconductor device shown in FIG. 13;

FIG. 15 is a sectional view schematically showing a second modificationof the semiconductor device shown in FIG. 13;

FIG. 16 is a sectional view schematically showing a seventh embodimentof a semiconductor device according to the present invention;

FIG. 17 is a sectional view schematically showing a first modificationof the semiconductor device shown in FIG. 16;

FIG. 18 is a sectional view schematically showing a second modificationof the semiconductor device shown in FIG. 16;

FIG. 19 is a sectional view schematically showing a third modificationof the semiconductor device shown in FIG. 16;

FIG. 20 is a sectional view schematically showing a fourth modificationof the semiconductor device shown in FIG. 16;

FIG. 21 is a sectional view schematically showing an eighth embodimentof a semiconductor device according to the present invention;

FIG. 22 is a sectional view schematically showing a modification of thesemiconductor device shown in FIG. 21;

FIG. 23 is a sectional view schematically showing a ninth embodiment ofa semiconductor device according to the present invention;

FIG. 24 is a perspective view schematically showing a modification ofthe semiconductor device shown in FIG. 23;

FIG. 25 is a sectional view schematically showing a tenth embodiment ofa semiconductor device according to the present invention;

FIG. 26 is a sectional view schematically showing a first modificationof the semiconductor device shown in FIG. 25;

FIG. 27 is a sectional view schematically showing a second modificationof the semiconductor device shown in FIG. 25;

FIG. 28 is a sectional view schematically showing a third modificationof the semiconductor device shown in FIG. 25;

FIG. 29 is a sectional view schematically showing a fourth modificationof the semiconductor device shown in FIG. 25;

FIG. 30 is a sectional view schematically showing an eleventh embodimentof a semiconductor device according to the present invention;

FIG. 31 is a sectional view schematically showing a first modificationof the semiconductor device shown in FIG. 30;

FIG. 32 is a sectional view schematically showing a second modificationof the semiconductor device shown in FIG. 30; and

FIG. 33 is a sectional view schematically showing a third modificationof the semiconductor device shown in FIG. 30.

DETAILED DESCRIPTION OF THE INVENTION

Several embodiments of the present invention will be describedhereinafter with reference to the drawings. In the followingdescription, an n-type is used as a first conductivity type, and ap-type is used as a second conductivity type. Moreover, in the followingdrawings the same parts are denoted with the same reference numerals andin the following description redundant description will be made onlywhen necessary.

First Embodiment

FIG. 1 is a sectional view schematically showing a first embodiment of asemiconductor device according to the present invention. A GaNheterostructure field effect transistor (hereinafter referred to simplyas an HFET) 220 which is shown in FIG. 1 comprises a channel layer 2, ann-type barrier layer 4, a p-type base layer 6, a gate electrode 16, asource electrode 12, and a drain electrode 14. The channel layer 2 isformed of an i-GaN layer, and corresponds to a first semiconductor layerrepresented by, for example, a composition formula Al_(x)Ga_(1-x)N(0≦x≦1). The n-type barrier layer 4 corresponds to a first conductivitytype or non-doped second semiconductor layer represented by, forexample, a composition formula Al_(y)Ga_(1-y)N (0≦y≦1, x<y), and isformed of an n-AlGaN layer on the channel layer 2 to supply an electronto a channel. Furthermore, the p-type base layer 6 is selectively formedof a p-GaN layer on the n-type barrier layer 4, and corresponds to athird semiconductor layer represented by, for example, the compositionformula Al_(x)Ga_(1-x)N (0≦x≦1). It is to be noted that the channellayer 2 is generally formed on a substrate of SiC, sapphire, Si, GaN orthe like which are not especially shown in FIG. 1.

The gate electrode 16 is formed on the base layer 6, and both of thesource electrode 12 and the drain electrode 14 are formed on the barrierlayer 4 so as to contact the barrier layer 4. The source electrode 12and drain electrode 14 form an ohmic contact with the barrier layer 4,and then an electron flows into the drain electrode 14 from the sourceelectrode 12 via a two-dimensional electron gas (2DEG) channel formed inan AlGaN/GaN hetero interface. These electrodes 12 and 14 can be formedof Ti/Al or the like.

In the HFET 220 of the present embodiment, the base layer 6 and barrierlayer 4 form a pn-junction, and therefore a gate leak current isdecreased as compared with a Schottky junction.

A gate threshold voltage is determined by a carrier concentration of2DEG channel. Therefore, to realize a normally-off state, when a gatevoltage is zero, a 2DEG carrier concentration is zero, specifically the2DEG channel has to be depleted. The 2DEG carrier concentration of theAlGaN/GaN heterostructure is determined by a sheet carrier concentrationof the barrier layer and a carrier concentration generated by piezopolarization generated by a stress of a hetero interface.

Since the HFET 220 of the present embodiment comprises the base layer 6formed of GaN on the barrier layer 4, the piezo polarization generatedin the hetero interface between the GaN channel layer 2 and the AlGaNbarrier layer 4 is canceled by that generated in the hetero interfacebetween the AlGaN barrier layer 4 and the GaN base layer 6. Accordingly,it is possible to selectively reduce the 2DEG carrier concentration of achannel portion.

Moreover, the p-GaN base layer 6 is formed so as to have a sheetimpurity concentration which is not less than that of the n-AlGaNbarrier layer 4. Accordingly, the 2DEG channel is depleted and thenormally-off state is realized.

As shown in FIG. 1, the base layer 6 is formed in a striped shape in thesame manner as in the gate electrode 16. Thus, it is possible to set anelectron concentration of the channel under the gate electrode 16 tozero when the gate voltage is 0 V, and the normally-off state can berealized.

In the conventional HFET, the normally-off state has been realized byreduction of thickness of the barrier layer 4. However, in this case,the 2DEG carrier concentration of a portion other than the channel alsodrops, and resistance of an offset portion between gate and source, orbetween gate and drain. As a result, an on-resistance has increased. Onthe contrary, in the HFET 220 of the present embodiment it is possibleto reduce the 2DEG carrier concentration only in a channel portion sincethe base layer 6 is selectively formed on the barrier layer 4. As aresult, the normally-off state is realized without increasing anyon-resistance.

A specific method of selectively forming the base layer 6 includes amethod in which a pattern is formed by etching after successiveimplementation of crystal growth on the channel layer 2, the barrierlayer 4 and the base layer 6, and a method in which an insulating filmis deposited to form a pattern after performing the crystal growth onthe channel layer 2 and the barrier layer 4, and then selective growth.

Furthermore, in the HFET 220 of the present embodiment, the distancebetween gate and drain Lgd1 is formed to be longer than the distancebetween gate and source Lgs1. Since the HFET is a horizontal typedevice, a breakdown voltage of the device is determined by that betweengate and drain. To obtain a device with the high breakdown voltage, thedistance between gate and drain needs to be lengthened. On the otherhand, the distance between gate and source, which may be a cause for aparasitic resistance, is preferably short regardless of the breakdownvoltage.

Moreover, increase of the breakdown voltage of the device can also beachieved by flattening the distribution of the electronic field betweengate and drain. An HFET which is realized by one of the specific meansis shown as a first modification of the HFET 220 shown in FIG. 1 in asectional view of FIG. 2. An HFET 222 comprises a first field plateelectrode 34 formed above the gate electrode 16 via a field insulatingfilm 32 so as to cover the gate electrode 16 and connected to the sourceelectrode 12 by a via hole. By this structure, an electric field in anend portion of the gate electrode 16 is defused, and the breakdownvoltage increases. It is to be noted that the electric field on the endportion of the gate electrode 16 can be defused even by the connectionof the first field plate electrode 34 to the gate electrode 16 withoutinterposing the field insulating film 32. However, in this case, thereis a disadvantage that a capacity between gate and drain increases and aswitching speed drops.

A second modification of the HFET 220 shown in FIG. 1 is shown in FIG.3. An HFET 224 shown in the drawing further comprises a second fieldplate electrode 36 connected to the drain electrode 14. Thus theelectric field in the end portion of the drain electrode 14 is similarlydefused, and it is possible to further increase the breakdown voltage.

Second Embodiment

FIG. 4 is a sectional view schematically showing a second embodiment ofa semiconductor device according to the present invention. A GaNinsulating gate type heterostructure field effect transistor(hereinafter referred to simply as GaN-MIS-HFET) 230 shown in FIG. 4further comprises a gate insulating film 22 formed so as to coat thebarrier layer 4 and base layer 6, and an insulating gate (MIS gate)structure including a gate electrode 18 formed on the gate insulatingfilm 22 instead of the gate electrode 16 of the GaN-HFET 220 shown inFIG. 1. The gate insulating film 22 can be formed of SiN, SiO2, Al2O3 orthe like.

According to the present embodiment, it is possible to set a gate leakcurrent substantially to zero by use of this MIS gate structure.

The other constitution of the GaN-MIS-HFET shown in FIG. 4 issubstantially the same as that of the HFET 220 shown in FIG. 1. The 2DEGcarrier concentration of the channel under the gate electrode when thegate voltage is 0 V can be set to zero by the base layer 6 formed of GaNin a stripe shape similar to that of the gate electrode 18 in the samemanner as in the first embodiment, and the normally-off state can berealized.

FIG. 5 shows a modification of a GaN-MIS-HFET 230 shown in FIG. 4. Inlocally forming the base layer 6 by the etching, even the barrier layer4 might be etched due to fluctuations in processes. In this case, the2DEG carrier concentration between the gate and source or between thegate and drain changes, and an on-resistance of the device changes sincethe thickness of the barrier layer 4 changes due to the etching of thebarrier layer 4.

A GaN-MIS-HFET 232 shown in FIG. 5 further comprises a buffer layer 8which is an i-GaN layer and is formed in a thickness exceeding thefluctuation of an etching depth and which is disposed between the baselayer 6 and the barrier layer 4. Thus, it is possible to suppress thefluctuation of the on-resistance by the etching fluctuation. The bufferlayer 8 corresponds to, for example, a fourth semiconductor layer, andis formed in the thickness exceeding the fluctuation of the etchingdepth. The barrier layer 4 is thus not etched. In addition, the sheetcarrier concentration in the channel does not change even when thethickness of the buffer layer 8 changes. Therefore, a constanton-resistance can be obtained.

Third Embodiment

FIG. 6 is a sectional view schematically showing a third embodiment of asemiconductor device according to the present invention.

In a GaN-MIS-HFET 240 shown in FIG. 6, the interval Lgd between the gateelectrode 18 and the drain electrode 14 is set to be larger than theinterval Lgs between the gate electrode 18 and the source electrode 12.A high breakdown voltage is required for a power semiconductor device,and the breakdown voltage needs to be held between the gate and drain inthe horizontal type device shown in FIG. 6. Therefore, when the distancebetween the gate and drain is lengthened, the breakdown voltage can beincreased.

FIG. 7 is a sectional view schematically showing a modification of theGaN-MIS-HFET 240 shown in FIG. 6. A GaN-MIS-HFET 242 shown in FIG. 7further comprises a first field insulating film 32 formed so as to coatthe gate electrode, and a field plate electrode 38 formed on the fieldinsulating film 32 and connected to the source electrode 12 by the viahole in order to obtain a higher breakdown voltage. When the gateelectrode 18 is covered by the field plate electrode 38, the electricfield in the end portion of the gate electrode 18 is defused, and thusthe breakdown voltage increases.

FIG. 8 is a sectional view schematically showing a second modificationof the GaN-MIS-HFET 240 shown in FIG. 6. A GaN-MIS-HFET 244 shown inFIG. 8 further comprises a second field plate electrode 42 formed on thedrain side in order to further increase the breakdown voltage. By thisstructure, the electric field in the end portion of the drain electrode14 is defused, and then the breakdown voltage increases. It is to benoted that in the example shown in FIG. 8, the field insulating film 32is formed in a uniform thickness, but when the thickness is changed instages, it is possible to further increase the breakdown voltage.

Fourth Embodiment

FIG. 9 is a sectional view schematically showing a fourth embodiment ofthe semiconductor device according to the present invention.

A GaN-MIS-HFET 250 shown in the drawing comprises a channel layer 2formed by crystal growth on a conductive semiconductor substrate 24, abarrier layer 4 and base layer 6, and a source electrode 12 electricallyconnected to the conductive semiconductor substrate 24 via aback-surface electrode 26. The substrate 24 is electrically connected tothe source electrode 12, and thus functions in the same manner as in thefield plate electrode 38 to defuse the electric field in the end portionof the gate electrode 18 or the drain electrode 14, and therefore thebreakdown voltage increases. An Si, SiC, or GaN substrate may be used asthe conductive semiconductor substrate. In the GaN-MIS-HFET 250 shown inFIG. 9, the back-surface electrode 26 is connected to the sourceelectrode 12 to electrically connect the source electrode 12 to thesubstrate 24. However, this embodiment is not limited to such aconstitution. The source electrode 12 can be connected to the substrate24 on the same surface as that of the source electrode 12 through, forexample, etching of the channel layer 2. A buffer layer for the crystalgrowth may also be disposed between the conductive semiconductorsubstrate 24 and the channel layer 2.

FIG. 10 is a sectional view schematically showing a modification of theGaN-MIS-HFET 250 shown in FIG. 9. Characteristics of a GaN-MIS-HFET 252shown in FIG. 10 lie in that the transistor further comprises a holeabsorption layer 44 formed of a p-layer having a high concentration onthe undersurface of the channel layer 2, and a hole absorption layer 46formed on the side of the same surface as that of the source electrode12 in the end portion of the hole absorption layer 44 and that the holeabsorption layer 44 is connected to the source electrode 12 via the holeabsorption layer 46. The hole absorption layer 46 corresponds to, forexample, a fifth semiconductor layer.

According to the present embodiment, when the channel layer 2, barrierlayer 4, and base layer 6 are successively formed on the hole absorptionlayer 44 formed of a p-GaN layer, the breakdown voltage increases, andadditionally it is possible to increase an avalanche withstandingcapability. In the structures of the HFET shown in FIGS. 1 to 5, when ahigh drain voltage is applied, and avalanche breakdown occurs, anelectron flows into the drain electrode 14, and a hole flows in the gateelectrode via the base layer 6. However, a layer which absorbs the holedoes not exist in the GaN-MIS-HFET structures shown in FIGS. 6 to 9.Therefore, there may be a disadvantage that the avalanche withstandingcapability is reduced.

Since the GaN-MIS-HFET 252 shown in FIG. 10 comprises a hole absorptionlayer 44 formed in a high-concentration p-layer under the channel layer2, it is possible to discharge the hole at a low resistance, and it isalso possible to increase the avalanche withstanding capability. Thehole absorption layer 44 is connected to the source electrode 12 andthus functions as the field plate electrode, and therefore the breakdownvoltage further increases. To securely flow the hole into the holeabsorption layer 44 at an avalanche breakdown time, the thickness of thechannel layer 2 is preferably set to be smaller than a distance Lgd2between gate and drain. In the example shown in FIG. 10, the holeabsorption layer 44 is electrically connected to the source electrode 12via the hole absorption layer 46 formed on the same surface side as thatof the source electrode 12, but an ohmic electrode of the holeabsorption layer 44 may also be taken out from the back surface, and thepresent invention is not limited by a way of taking out the electrode.

It is to be noted that it is possible to increase the avalanchewithstanding capability in the HFET structure shown in FIGS. 1 to 5 byadditionally disposing the hole absorption layer.

Fifth Embodiment

FIG. 11 is a perspective view schematically showing the constitution ofthe semiconductor device according to a fifth embodiment of the presentinvention. The characteristics of a GaN-MIS-HFET 260 shown in thedrawing lie in that the transistor comprises a base electrode 50connected to the source electrode 12, and thus a carrier is quicklydischarged in a case where the base layer is charged.

Since each of the semiconductor devices shown in FIGS. 6 to 10 has a MISgate structure, the base layer 6 is not connected to any electrode, anda potential of the base layer 6 has a state similar to that of afloating electrode. Therefore, when the electron is injected into thebase layer 6 and the hole is discharged from the base layer 6, the baselayer 6 remains to be charged. On the other hand, when the base layer isformed so as to cover the whole surface of a region between the sourceelectrode 12 and the gate electrode 18, a resistance between gate andsource increases. A base layer 56 of the GaN-MIS-HFET 260 shown in FIG.11 has an extended portion formed in the striped shape so that the baselayer is locally connected to the source electrode 12 and extends in adirection orthogonal to a longitudinal direction of the source electrode12 and is connected to the source electrode 12 by this extended portion.A portion of the base layer 56 formed along the longitudinal directionof the source electrode 12 corresponds to, for example, a first stripeportion. The extended portion of the base layer 56 formed in the stripedshape in a direction orthogonal to the longitudinal direction of thesource electrode 12 corresponds to, for example, a second stripeportion.

Since the base layer 56, barrier layer 4, and channel layer 2 arebasically formed in a p-GaN/n-AlGaN/i-GaN heterostructure, any carrieris not injected into the base layer 56 as long as a large voltage isapplied to the gate voltage to pass the current by a tunnel effect, buta large voltage is sometimes applied to the gate electrode for certaincauses such as a noise at a switching time. According to the presentembodiment, since the transistor comprises the base layer 56 having theshape shown in FIG. 11, it is possible to quickly discharge the carrier.

It is to be noted that since the base layer 56 is connected to thesource electrode 12, a channel portion is preferably depleted in orderto control the potential of the portion by the voltage applied to thegate electrode 18.

FIG. 12 shows a modification of the GaN-MIS-HFET 260 shown in FIG. 11. Ap-base layer 58 disposed in a GaN-MIS-HFET 262 of the present example isconnected to the source electrode 12 in the same manner as in theMIS-HFET 260 of FIG. 11, but a portion of the p-base layer 58 under thegate electrode 18 does not have a continuous striped shape, and isdiscontinuously formed. This structure can realize a stable gatethreshold voltage even in a case where the p-base layer 58 does not havea strictly controlled impurity concentration. When the p-base layer isformed in the continuous shape as the p-base layer 56 shown in FIG. 11,the gate threshold voltage changes dependently on the impurityconcentration of the p-base layer 56. Furthermore, the impurityconcentration of the p-base layer 56 has to be lowered to a certaindegree in order to change the potential of the p-base layer 56 by thegate voltage.

Then, when the p-base layer 58 is discontinuously formed as shown inFIG. 12, a portion in which the p-base layer 58 is not formedselectively forms the channel. In this case, when the impurityconcentration of the p-base layer 58 is sufficiently raised, the gatethreshold voltage is determined by an interval WB3 between the segmentedportions of the p-base layer 58 under the gate electrode 18 which areadjacent to each other, not by the impurity concentration. Moreover,since dimensional precision of the interval WB3 is determined by adimension system of lithography, the fluctuation is small, and a stablegate threshold voltage can be realized.

The interval WB3 needs to be reduced so that the gate threshold voltageis positive, and the normally-off state is realized. Furthermore, sinceany channel is not formed in the extended portion of the p-base layer 58in an offset region between the gate and source, an interval WBI of theextended portion of the p-base layer 58 needs to be broadened in orderto secure a broader region of a depletion layer and to lower resistanceof the offset region between the gate and source. On the other hand, awidth WB2 of the extended portion needs to be reduced. The width WB2 ofthe extended portion is preferably a half of mutual pitches((WB1+WB2)/2) or less.

It is to be noted that for the structure shown in FIGS. 11, 12, it isalso possible to increase the breakdown voltage by additionallydisposing the field plate electrode, and furthermore, it is alsopossible to enhance the avalanche withstanding capability by adding thehole absorption layer.

Sixth Embodiment

FIG. 13 is a sectional view schematically showing a configuration of asixth embodiment of the semiconductor device according to the presentinvention.

The characteristics of an HFET 270 shown in the drawing lie in a recessgate structure comprising a barrier layer 64 formed on the channel layer2 so that only a region under the gate electrode is thin, and in thatthe base layer 6 is disposed on the bottom surface of the recess portionof the barrier layer 64. The barrier layer 64 corresponds to, forexample, a first conductivity type or non-doped second semiconductorlayer, and the base layer 6 corresponds to, for example, a thirdsemiconductor layer. According to the present embodiment, an electronconcentration is set to zero only in the channel portion under the gateby locally changing the thickness of the barrier layer 64. Therefore,the normally-off state is realized with the on-resistance remaining tobe low.

In order to realize the normally-off state, the electron concentrationof the channel layer can be made zero by setting the sheet impurityconcentration of the base layer 6 larger than that of the thin portionof the barrier layer 64 under the gate electrode 16, the thin portiondetermining the electron concentration of the channel. On the otherhand, since the electron concentration of the channel layer is notrendered to zero in a thick portion of the barrier layer 64 other thanthe recess portion thereof, the low on-resistance is realized.

FIG. 14 is a sectional view showing a first modification of the HFET 270shown in FIG. 13. An HFET 272 shown in FIG. 14 comprises a p-base layer66 formed so as to extend not only to a portion right under the gateelectrode 16 in which the barrier layer 64 is formed to be thin but alsoa portion in which the barrier layer 64 is thickened via the sidesurface of a recess. In this manner, the p-base layer 66 may also beformed so as to extend to not only the recess bottom surface but also aportion in which the barrier layer 64 is thickened. FIG. 15 is asectional view showing a second modification of the HFET 270 shown inFIG. 13. An HFET 274 shown in the drawing comprises a base layer 68extending toward a drain electrode side. The base layer 68 is preferablydisposed to be as long as possible to coat the thick portion of thebarrier layer 64 up to a region on the verge of the drain electrode 14,and the extended portion is preferably formed so as to coat at least thehalf or more of the thick portion of the barrier layer 64. By this shapeof the base layer 68, the depletion layer of the barrier layer 64quickly extends toward a drain electrode 14 even in a case where a highvoltage is applied to the drain electrode. Accordingly, an effectsimilar to that of an RESURF layer is obtained, the distribution inelectric field between the gate and drain becomes flat, and a higherbreakdown voltage can be obtained.

Seventh Embodiment

FIG. 16 is a sectional view schematically showing the constitution of aseventh embodiment of the semiconductor device according to the presentinvention.

A GaN-MIS-HFET 280 shown in the drawing comprises an insulating gate(MIS gate) structure including a gate insulating film 82 formed so as tocoat the barrier layer 64 and base layer 6, and a gate electrode 72formed on the gate insulating film 82 instead of the gate electrode 16of the HFET 274 shown in FIG. 15.

According to the present embodiment, the gate leak current can bereduced by use of this MIS gate structure. When the sheet impurityconcentration of the base layer 6 is set to be larger than that in athinned region of the barrier layer 64 under the gate electrode 72, thenormally-off state can be realized in the same manner as in thestructure of the HFET 270 shown in FIG. 13.

FIG. 17 is a sectional view showing a first modification of theGaN-MIS-HFET 280 shown in FIG. 16. As in a GaN-MIS-HFET 282 shown in thedrawing, the p-base layer 68 may also be formed so as to extend to thethick portion of the barrier layer 64. The extended portion of the baselayer 68 is preferably formed to coat the half or more of the regionbetween the gate electrode 72 and the drain electrode 14 in a surfaceregion of the barrier layer 64. Furthermore, in a second modificationshown in FIG. 18, an interval Lgd3 between the gate electrode 72 and thedrain electrode 14 is set to be larger than an interval Lgs3 between thegate electrode 72 and the source electrode 12. For a power semiconductordevice, a high breakdown voltage is required, and the breakdown voltageneeds to be held between the gate and drain in the horizontal typedevice. According to a GaN-MIS-HFET 284 of the present example, it ispossible to increase the breakdown voltage by lengthening the distancebetween gate and drain.

FIG. 19 shows a third modification comprising a structure for obtaininga higher breakdown voltage. A GaN-MIS-HFET 286 shown in the drawingcomprises a field insulating film 92 formed so as to coat the gateelectrode 72, and a field plate electrode 94 formed on this fieldinsulating film 92. Since the gate electrode 72 is covered with thefield plate electrode 94 in this manner, the electric field in the endportion of the gate electrode 72 is defused, and the breakdown voltageincreases.

A fourth modification comprising a structure for further increasing thebreakdown voltage is shown in FIG. 20. The characteristics of aGaN-MIS-HFET 288 shown in the drawing lie in that the transistor furthercomprises a second field plate electrode 96 formed on the drain side ofthe field insulating film 92. Thus, the electric field in the endportion of the drain electrode 14 is defused, and the breakdown voltagefurther increases. It is to be noted that in FIG. 20, the fieldinsulating film 92 formed in a uniform thickness is shown, it ispossible to further increase the breakdown voltage even when thethickness is changed in stages.

Eighth Embodiment

FIG. 21 is a sectional view schematically showing a constitution of aneighth embodiment according to the present invention. Thecharacteristics of a MIS-HFET 290 shown in the drawing lie in that thechannel layer 2, the barrier layer 64, and the base layer 68 are formedabove the conductive semiconductor substrate 24, and the conductivesemiconductor substrate 24 is connected to the source electrode 12 viathe back-surface electrode 26. Thus, the substrate 24 functions as afield plate, and the distribution in the electric field between gate anddrain approaches a flat distribution, and it is therefore to increasethe breakdown voltage. As a result, a breakdown voltage further higherthan that of the GaN-MIS-HFET 286 shown in FIG. 19 can be realized.

FIG. 22 shows a modification of the MIS-HFET 290 shown in FIG. 21. AMIS-HFET 292 comprises a hole absorption layer 44 formed of a p-type GaNlayer beneath the channel layer 2 instead of the field plate structuredisposed below the channel layer. Accordingly, the breakdown voltage isincreased in the same manner as in the lower field plate structure, theholes can be quickly discharged, and it is also possible to increase theavalanche withstanding capability.

Ninth Embodiment

FIG. 23 is a sectional view schematically showing a constitution of aninth embodiment of the semiconductor device according to the presentinvention.

Since the devices shown in FIGS. 16 to 22 have the MIS gate structure,the base layer is not connected to any electrode, and therefore eachbase layer becomes similar to a floating electrode. Therefore, when theelectron is injected into the base layer and the hole is discharged fromthe base layer, the base layer remains to be charged.

A MIS-HFET 300 shown in FIG. 23 comprises a base layer 74 connected tothe source electrode 12. By this structure, the carrier is quicklydischarged when the base layer is charged. On the other hand, unlike theMIS-HFET 260 shown in FIG. 11, even when a base layer 74 is formed so asto coat the whole surface of the region between the source electrode 12and the gate electrode 72, the barrier layer 64 other than the channelportion is thick, and therefore the resistance between gate and sourcedoes not increase, and the on-resistance does not increase.

Moreover, the base layer 74 is connected to the source electrode 12 inorder to control the potential of the channel portion by the voltageadded to the gate electrode 12, but the impurity concentration of thebase layer 74 is preferably small to such an extent that the channelportion is depleted by the carrier of the thick barrier layer 64 betweenthe gate and source.

FIG. 24 is a perspective view schematically showing a modification ofthe MIS-HFET 300 shown in FIG. 23. According to a MIS-HFET 302 shown inFIG. 24, since a base layer 75 is locally formed right under the gateelectrode 72, the impurity concentration of the p-base layer 75 is highin the same manner as in the MIS-HFET 260 shown in FIG. 12. Even whenany strict concentration control is not performed, the device can berealized without any increase of the on-resistance or any change of thegate threshold voltage.

Moreover, also in the structures shown in FIGS. 23 and 24, the breakdownvoltage can be increased, when the field plate electrode is additionallyarranged. When the hole absorption layer is additionally provided, theavalanche withstanding capability can be increased.

Tenth Embodiment

FIG. 25 is a sectional view schematically showing a constitution of atenth embodiment of the semiconductor device according to the presentinvention.

The characteristics of an HFET 310 shown in FIG. 25 lie in that thetransistor comprises a channel layer 102 formed of a p-type GaN layerand accordingly the gate threshold voltage shifts on a plus side. Thechannel layer 102 corresponds to, for example, a second conductivitytype second semiconductor layer. Moreover, the barrier layer 64 has therecess gate structure in which only a region for the gate electrode isformed to be thin. Therefore, the 2DEG carrier concentration is reducedonly in the region right under the gate electrode 16, and thenormally-off state is easily realized. On the other hand, since the 2DEGcarrier concentration is large in the offset portion between the gateand drain or between the gate and source, the on-resistance is small.The barrier layer 64 of the present embodiment corresponds to, forexample, the first conductivity type or non-doped first semiconductorlayer.

In this structure of the HFET 310, generation of a 2DEG carrier by piezopolarization in an AlGaN/GaN hetero interface cannot be inhibited ascompared with a case where the p-GaN layer is disposed on the barrierlayer 64 as in the HFET 270 shown in FIG. 13. Therefore, there is adisadvantage that the normally-off state is not realized unless thesheet impurity concentration of the p-type channel layer 102 is set tobe further larger than that of the barrier layer 64 by a charge by thepiezo polarization. However, the HFET 310 of the present embodiment doesnot require any step of re-growth after forming the recess gate, andtherefore there is an advantage that the structure can be formed by onecrystal growth.

FIG. 26 is a sectional view schematically showing a first modificationof the HFET 310 shown in FIG. 25. An HFET 312 shown in the drawingfurther comprises the channel layer 2 formed of i-GaN under the p-typechannel layer 102. Since the gate threshold voltage is determined by thesheet impurity concentrations of the barrier layer 64 and p-type channellayer 102, the p-type channel layer may be formed only in the vicinityof the hetero interface.

FIG. 27 is a sectional view schematically showing a second modificationof the HFET 310 shown in FIG. 25. The characteristics of an HFET 314shown in the drawing lie in that the p-type channel layer 102 isconnected to a source electrode 104. By this structure, since the holegenerated at an avalanche breakdown is quickly discharged, the avalanchewithstanding capability is increased. However, when the impurityconcentration of the p-type channel layer 102 is excessively high in thestructure shown in FIG. 27, the channel layer is not depleted when thevoltage is applied, the avalanche breakdown occurs at a low voltage, andthere is a possibility that the breakdown voltage drops. To solve theproblem, it is preferable to use the structure in which the p-typechannel layer 102 is also depleted so as to obtain a high breakdownvoltage when the voltage is applied to the layer. Specifically, thesheet impurity concentration of the p-type channel layer 102 is set tobe substantially equal to that of the barrier layer 64.

FIG. 28 is a sectional view schematically showing a third modificationof the HFET 310 shown in FIG. 25. In addition to the structure shown inFIG. 27, an HFET 316 shown in FIG. 28 further comprises a field plateelectrode 34 which is formed so as to cover the gate electrode 16 viathe field insulating film 32 and which is connected to the sourceelectrode 104. By this structure, since the electric field in the endportion of the gate electrode 16 is defused, it is possible to increasethe breakdown voltage of the device. Furthermore, even when the fieldplate electrode 36 is additionally disposed on the drain side on thefield insulating film 32 as in an HFET 318 shown in FIG. 29, it ispossible to further increase the breakdown voltage.

Eleventh Embodiment

FIG. 30 is a sectional view schematically showing a constitution of aneleventh embodiment of the semiconductor device according to the presentinvention. The characteristics of a MIS-HFET 320 shown in the drawinglie in that the transistor further comprises a gate insulating film 84formed on the surface of the barrier layer 64 and that the gateelectrode 16 is formed in a concave portion of the barrier layer 64 viathe gate insulating film 84. The present invention is also applicable tosuch MIS gate structure. Furthermore, when the field plate electrode 34is disposed so as to cover the gate electrode 16 via the fieldinsulating film 32 as in modifications 322 and 324 shown in FIGS. 31 and32, respectively, a field concentration in the end of the sourceelectrode 12 or the drain electrode 14 is defused, and it is possible tofurther increase the breakdown voltage.

Furthermore, when the p-type channel layer 102 is connected to thesource electrode 104 as in a MIS-HFET 326 shown in FIG. 33, the hole canbe quickly discharged, and it is therefore possible to increase theavalanche withstanding capability.

The first to eleventh embodiments of the present invention have beendescribed above, but the present invention is not limited to theabove-described embodiments, and any person skilled in the art caneasily devise various other modifications within the scope of thepresent invention.

For example, the channel layer 2, base layer 6, and buffer layer 8 havebeen described using the GaN layer, but an AlGaN layer can beimplemented, when an equal Al composition ratio is set to each of threelayers, and set to be smaller than that of the barrier layer 4.Moreover, with respect to the Al composition ratio of the buffer layer8, the channel layer 2 is equal to the base layer 6, but differentcomposition ratios may also be used. However, the Al composition ratioof the buffer layer 8 is preferably equal to that of the channel layer 2in order to cancel the piezo polarization in the hetero interfacebetween the barrier layer 4 and the channel layer 2 and to lower the2DEG carrier concentration under the gate electrode.

Moreover, the invention can be implemented as long as a relation betweenband gaps is the same even when the band gaps are changed by acomposition ratio such as a case where an InGaN layer is used for thechannel layer and a GaN layer is used for the barrier layer and a casewhere an AlGaN layer is used for the channel layer and an AlN layer isused for the barrier layer.

Furthermore, an i-AlGaN layer may also be inserted between the channellayer 2 and the barrier layer 4 or between the barrier layer 4 and thebase layer 6 in order to keep steepness of modulation doping or heterointerface.

Additionally, the semiconductor layers such as the channel layer andbarrier layer may be formed by the crystal growth on the substrate, andmay be implemented with substrates such as GaN, SiC, sapphire, and Si,but the present invention is not limited to the material of thesubstrate, and a buffer layer or the like involved by the crystal growthmay also be formed under the channel layer.

Moreover, in the present invention, a threshold voltage can be shiftedon a plus side even in a normally-on device by forming the GaN layer onthe AlGaN barrier layer for the purpose of realizing the normally-offstate. Especially, the Al composition ratio of the base layer 6 is equalto that of the channel layer 2. However, in a meaning that the carrierby the piezo polarization is reduced, even when the Al composition ratiois not equal, the present invention may be implemented with an Alcomposition ratio smaller than that of the barrier layer 4.

1. A semiconductor device comprising: a first semiconductor layerrepresented by a composition formula Al_(x)Ga_(1-x)N (0≦x≦1); a firstconductivity type or non-doped second semiconductor layer represented bya composition formula Al_(y)Ga_(1-y)N (0≦y≦1, x<y), formed on the firstsemiconductor layer and having a concave portion; a second conductivitytype third semiconductor layer represented by a composition formulaAl_(x)Ga_(1-x)N (0≦x≦1) and is selectively formed above a bottom surfaceof the concave portion of the second semiconductor layer; a gateelectrode formed on the third semiconductor layer; a source electrodeelectrically connected to the second semiconductor layer; and a drainelectrode electrically connected to the second semiconductor layer,wherein the third semiconductor layer is formed so as to coat the bottomsurface of the concave portion, a side surface of the concave portionand a surface of a portion of the second semiconductor layer which isthicker than the concave portion.
 2. The semiconductor device accordingto claim 1, wherein the third semiconductor layer is formed so as tocoat at least half a surface region of the second semiconductor layerbetween the gate electrode and the drain electrode.
 3. The semiconductordevice according to claim 1, which further comprises a gate insulatingfilm formed so as to be sandwiched by the third semiconductor layer andthe gate electrode.
 4. A semiconductor device comprising: a firstsemiconductor layer represented by a composition formula Al_(x)Ga_(1-x)N(0≦x≦1); a first conductivity type or non-doped second semiconductorlayer represented by a composition formula Al_(y)Ga_(1-y)N (0≦y≦1, x<y),formed on the first semiconductor layer and having a concave portion; asecond conductivity type third semiconductor layer represented by acomposition formula Al_(x)Ga_(1-x)N (0≦x≦1) and is selectively formedabove a bottom surface of the concave portion of the secondsemiconductor layer; a gate electrode formed on the third semiconductorlayer; a source electrode electrically connected to the secondsemiconductor layer; and a drain electrode electrically connected to thesecond semiconductor layer, wherein the third semiconductor layer isformed so as to extend to contact the source electrode through the sidesurface of the concave portion.